Air strippers generally partition volatile compounds found within contaminated liquids, such as groundwater, between liquid and gas phases by bringing the liquid phase into intimate contact with a stream of air. Volatile contaminants are removed from the liquid as the air is discharged from the air stripper.
Partitioning of volatile compounds between gas and liquid phases is a mass transfer process that is closely defined by Henry's law. According to Henry's law, the equilibrium concentration of a volatile compound in the water phase is proportional to the partial pressure of the compound in the air that is in contact with the water. Absent any of the volatile compound in the air and given adequate time, migration will proceed from the liquid phase into the gas phase until equilibrium is reached. As the temperature increases at a fixed pressure, the Henry's law constant of proportionality for a given volatile compound increases, meaning that the equilibrium concentration for the compound in the water phase decreases and the equilibrium concentration in the air phase increases. Thus, higher operating temperatures within air strippers increase both the rate of migration due to the increase in the Henry's law constant (the driving force for the mass transfer) and the percentage of the original mass of the volatile compound within the liquid that can migrate to the air stream as the system approaches equilibrium.
Several conventional types of air stripping methods that are commonly used include, for example, packed towers, diffused aeration, tray aeration, and spray aeration. Of these, packed towers and diffused aeration are the most common methods. The designs of each of these types of air strippers include means to create zones where the gas and liquid are brought into intimate contact over a large interfacial surface area. The particular method used to create interfacial surface area affects the size of the air stripper and compact designs are generally more desirable considering the value of floor and air space within industrial facilities and the negative aesthetic impact of larger units when installed outdoors.
Typical packed tower air strippers include a spray nozzle or weir at the top of a tower to uniformly distribute contaminated water over a column of packing, a fan to force air counter current to the water flow, and a sump at the bottom of the tower to collect decontaminated water. The packed tower type air stripper thus increases the interfacial surface area between the water and air by distributing the water as a downward flowing film on the extended surface of the packing material. Accordingly, packed tower type air strippers generally require regular maintenance (to clean the contact surfaces) and occasionally become clogged with suspended matter that is either carried in with the feed liquid or formed by compounds that precipitate out of the water. Clogged passageways in packing can also create conditions that stimulate biological growth further compounding the problem of clogging. An additional drawback to the packed tower type air stripper is that a balance must be struck between the amount of void space in the packed column and the restriction that the packing presents to the flow of air in the gas-liquid contact zone. The void space creates a tortuous path that forces the gas into intimate contact with the liquid film flowing over the packing. Smaller void space increases the velocity of the gas over the liquid film and enhances turbulence, which favorably affects the rate of mass transfer from the liquid to the gas phase. Because a finite amount of void space must be employed and additional space is occupied by the mass of packing, the volume of the gas-liquid contact zone of a packed tower is larger than the required volume of the gas-liquid contact zone within air strippers where the gas is introduced directly into the liquid phase, such as diffused aeration. In typical packed columns this additional volume compared to diffused aeration is reflected in increased vertical height. For example, to achieve approximately 99% efficiency, a typical packed tower type air stripper would require 15 to 20 feet of conventional packing. This can lead to space problems in certain situations.
On the other hand, diffused aeration devices, typically in the form of aeration tanks, are generally fairly low profile devices. In a typical aeration tank air bubbles are introduced into a tank of contaminated liquid through a distribution manifold that often includes small openings and/or diffuser devices such as screens that are usually located near the bottom of the tank and are designed to disperse the gas as uniformly as a possible throughout the liquid. Baffles and multiple gas distribution units may be used to ensure adequate dispersion of air bubbles and residence time for stripping to occur. Aeration tanks are prone to problems similar to the packed tower type systems in that the openings in the manifold and/or diffuser devices of an aeration tank may become clogged with suspended solids and/or biological growth. This problem is compounded by the fact that maximizing the interfacial surface area between gas and liquid requires minimizing bubble size (i.e., smaller opening in the manifold and/or diffuser screens).
Other than potential fouling and the need for periodic cleaning, traditional packed tower and diffused aeration type air strippers are generally good for remediation of liquids that are contaminated with volatile or semi-volatile organic compounds. To increase the rate and percentage of mass transfer of such compounds, the contaminated liquid is often preheated prior to treatment. Accordingly, the energy costs of such systems are quite high.
One variation of a diffused aeration type of air stripper is the submerged gas evaporators, also known as submerged gas reactors and/or combination submerged gas evaporator/reactor systems, in which gas is dispersed within a liquid. U.S. Pat. No. 5,342,482, which is hereby incorporated by reference, discloses a common type of submerged gas evaporator, in which combustion gas is generated and delivered though an inlet pipe to a dispersal unit submerged within the liquid to be evaporated. The dispersal unit includes a number of spaced-apart gas delivery pipes extending radially outward from the inlet pipe, each of the gas delivery pipes having small holes spaced apart at various locations on the surface of the gas delivery pipe to disperse the combustion gas as small bubbles as uniformly as practical across the cross-sectional area of the liquid held within the processing vessel. According to current understanding within the prior art, this design provides desirable intimate contact between the liquid and the combustion gas over a large interfacial surface area while also promoting thorough agitation of the liquid within the processing vessel.
Because submerged gas evaporators/reactors do not require heat exchangers with solid heated surfaces to raise the operating temperature of the process, these processors provide a significant advantage compared to conventional air strippers when contact between a heated liquid stream and a gas stream is desirable.
Suspended solids that may be carried into the air stripper with the contaminated fluid and/or particles that may precipitate from the liquid undergoing processing can form deposits on extended surfaces of liquid distribution devices used in conventional air strippers. Buildup of deposits on these extended surfaces and the possible formation of large crystals of precipitates and/or agglomerates related to solid particles can block passages within processing equipment such as passages in gas distribution manifold openings and diffuser devices used in diffused aeration systems or in the system described in U.S. Pat. No. 5,342,482. Such deposits and blockages reduce the efficiency of the air stripper and necessitate frequent cleaning cycles to avoid sudden unplanned shutdowns of the air stripper.
Additionally, most air stripping systems are prone to problems related to carryover of entrained liquid droplets that are swept from the liquid phase into the gas phase as the gas passes over and disengages from the liquid phase. For this reason, most air stripper systems include one or more devices to minimize entrainment of liquid droplets and/or to capture entrained liquid droplets (e.g., demisters) while allowing for separation of the entrained liquid droplets from the exhaust gas. The need to mitigate carryover of entrained liquid droplets may be related to one or more factors including conformance with environmental regulations, conformance with health and safety regulations and controlling losses of material that might have significant value.
Unlike conventional packed tower and tray type air stripping systems where mass is transferred from the liquid being processed to the air stream at locations along extended surfaces within the air stripper, mass transfer within submerged gas and diffused aeration evaporators/reactors takes place at the interfacial surface area between a discontinuous gas phase dispersed within a continuous liquid phase. Compared to the fixed extended surfaces employed in conventional packed tower and tray type air stripping systems, there are no extended solid surfaces within submerged gas and diffused aeration processors. Thus, because submerged gas processors and diffused aeration tank air strippers in general rely on dynamic renewable interfacial surface area that is constantly being formed between liquid and gas phases, the problem of deposits forming on extended surfaces is eliminated. The dynamic interfacial surface area that is constantly renewed by a steady flow of gas into the liquid phase of submerged gas and diffused aeration tank air strippers allows the air and liquid phases to remain in contact for a finite period of time before disengaging. This finite period of time is called the residence time of the gas within the evaporation, or evaporation/reaction zone.
Submerged gas and diffused aeration evaporators/reactors also tend to mitigate the formation of large crystals of compounds that precipitate from the liquid phase because dispersing the gas beneath the liquid surface provides mixing within the evaporation or the evaporation/reaction zone, which is a less desirable environment for crystal growth than a more quiescent zone. Further, active mixing within an evaporation or reaction vessel tends to maintain solid particles in suspension and thereby mitigates blockages that are related to settling and/or agglomeration of suspended solids.
However, mitigation of crystal growth and settlement or agglomeration of suspended solids is dependent on the degree of mixing achieved within a particular submerged gas or diffused aeration evaporator/reactor, and not all submerged gas or diffused aeration evaporator/reactor designs provide adequate mixing to prevent large crystal growth and related blockages. Therefore, while the dynamic renewable interface feature of submerged gas and diffused aeration evaporators/reactors eliminates the potential for fouling liquids to coat extended surfaces, conventional submerged gas and diffused aeration evaporators/reactors are still subject to potential blockages of small openings in the devices used to disperse gas into liquid.
Direct contact between hot gas and liquid undergoing processing within a submerged gas evaporator/reactor provides excellent heat transfer efficiency. If the residence time of the gas within the liquid is adequate for the gas and liquid temperatures to reach equilibrium, a submerged gas evaporator/reactor operates at a very high level of overall energy efficiency. For example, when hot gas is dispersed in a liquid that is at a lower temperature than the gas and the residence time is adequate to allow the gas and liquid temperatures to reach equilibrium at the adiabatic saturation temperature for the system, all of the available driving forces to affect mass and heat transfer, and allow chemical and physical changes to proceed to equilibrium stages, will have been consumed within the process. The minimum residence time to attain equilibrium of gas and liquid temperatures within the evaporation, reaction or combined reaction/evaporation zone of a submerged gas evaporator/reactor is a function of factors that include, but are not limited to, the temperature differential between the hot gas and liquid, the properties of the gas and liquid phase components, the properties of the resultant gas-liquid mixture, the net heat absorbed or released through any chemical reactions and the extent of interfacial surface area generated as the hot gas is dispersed into the liquid.
Given a fixed set of values for temperature differential, properties of the gas and the liquid components, properties of the gas-liquid mixture, heats of reaction and the extent of the interfacial surface area, the residence time of the gas is a function of factors that include the difference in specific gravity between the gas and liquid or buoyancy factor, and other forces that affect the vertical rate of rise of the gas through the liquid phase including the viscosity and surface tension of the liquid. Additionally, the flow pattern of the liquid including any mixing action imparted to the liquid such as that created by the means chosen to disperse the gas within the liquid affect the rate of gas disengagement from the liquid.
Submerged gas evaporators/reactors may be built in various configurations. One common type of submerged gas evaporator/reactor is the submerged combustion gas evaporator that generally employs a pressurized burner mounted to a gas inlet tube that serves as both a combustion chamber and as a conduit to direct the combustion gas to a dispersion system located beneath the surface of liquid held within an evaporation vessel. The pressurized burner may be fired by any combination of conventional liquid or gaseous fuels such as natural gas, oil or propane; any combination of non-conventional gaseous or liquid fuels such as biogas (e.g., landfill gas) or residual oil; or any combination of conventional and non-conventional fuels.
Other types of submerged gas evaporators/reactors include hot gas evaporators where hot gas is either injected under pressure or drawn by an induced pressure drop through a dispersion system located beneath the surface of liquid held within an evaporation vessel. While hot gas evaporators may utilize combustion gas such as hot gas from the exhaust stacks of combustion processes, gases other than combustion gases or mixtures of combustion gases and other gases may be employed as desired to suit the needs of a particular evaporation process. Thus, waste heat in the form of hot gas produced in reciprocating engines, turbines, boilers or flare stacks may be used within hot gas evaporators. In other forms, hot gas evaporators may be configured to utilize specific gases or mixtures of gases that are desirable for a particular process such as air, carbon dioxide or nitrogen that are indirectly heated within heat exchangers prior to being injected into or drawn through the liquid contained within an evaporation vessel.
Regardless of the type of submerged gas evaporator/reactor or the source of the gas used within the submerged gas evaporator/reactor, in order for the process to continuously perform effectively, reliably and efficiently, the design of the submerged gas evaporator/reactor must include provisions for efficient heat and mass transfer between gas and liquid phases, control of entrained liquid droplets within the exhaust gas, mitigating the formation of large crystals or agglomerates of particles and maintaining the mixture of solids and liquids within the submerged gas evaporator/reactor vessel in a homogeneous state to prevent settling of suspended particles carried within the liquid feed and/or precipitated solids formed within the process.